Dzhokhar Tsarnaev coolly dropped off a homemade bomb in the crowd at the Boston Marathon and then walked away amid the confusion as another explosive set by his brother detonated nearby, federal authorities alleged Monday in charging the teen with a terrorist act that could carry the death penalty.

Investigators are invoking the "public safety exception" in their questioning of Boston Marathon bombing suspect Dzhokhar Tsarnaev before advising him of his rights. Will using it now make things more difficult for prosecutors later? WSJ's Jason Bellini has "The Short Answer." Image: Associated Press/ Bob Leonard

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SAYING GOODBYE: Mourners left the funeral of Krystle Campbell, one of the victims of the Boston Marathon bombings, a week after the terrorist attack. Moments of silence were observed around the country.
Kevork Djansezian/Getty Images

In a criminal complaint unsealed in Boston federal court, prosecutors alleged that Mr. Tsarnaev, a 19-year-old college sophomore, detonated a weapon of mass destruction—a homemade bomb—in an attack that killed three and injured more than 200 on April 15. The complaint offered new details on the young man's alleged actions during the bombing, as well as the chaotic chase days later that ended in a shootout and the death of his 26-year-old older brother, Tamerlan.

The younger brother has been communicating with Federal Bureau of Investigation interrogators, according to a law-enforcement official, although the value of his written answers to questions wasn't immediately clear. Mr. Tsarnaev can't speak due to a gunshot wound to his throat which may have been self-inflicted, officials said.

Court Documents

Read the full criminal complaint against Dzhokhar Tsarnaev, a suspect in the Boston Marathon bombings.

Mr. Tsarnaev's brief legal proceeding was held Monday with a federal judge at the hospital because the suspect's injuries are so severe. He had also suffered gunshot wounds to the head, legs and hand when captured Friday night, according to an affidavit from FBI special agent Daniel Genck.

Justice Department spokesman Dean Boyd said the government "will always seek to elicit all the actionable intelligence and information we can from terrorist suspects taken into our custody."

Authorities said Monday that nothing they have learned so far leads them to believe there are more bombs in the Boston area, or other would-be plotters.

Former Manhattan Prosecutor Robert Viteretti joins The News Hub as the U.S. charges Boston bombing suspect Dzhokhar Tsarnaev to explain what happens next. Photo: Associated Press.

Anwar Kazmi, a board member of the Islamic Society of Boston, describes visits that bombing suspect Tamerlan Tsarnaev made to the Society's center. Photo: Associated Press.

The video and photos provided by federal prosecutors showed the two brothers carrying backpacks as they walk onto Boylston Street, the scene of the bombings, about 11 minutes before the first blast occurred at 2:49 p.m., the affidavit said.

At about 2:42 p.m., Tamerlan Tsarnaev, accused of planting the bomb that caused the first blast nearest the finish line, left his brother and was seen carrying the backpack that investigators believe contained the first bomb, according to the affidavit.

Three minutes later, Dzhokhar Tsarnaev walked toward the Forum restaurant and left a backpack on the ground, according to video surveillance cited in the affidavit. About 30 seconds before the first bomb exploded, video footage shows him looking at his cellphone and then lifting the phone to his ear as if to speak, the FBI affidavit said.

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A few seconds after his apparent call, the first explosion a block away drew the crowd's attention, but Dzhokhar Tsarnaev, "virtually alone among the individuals in front of the restaurant, appears calm," the affidavit said.

He glanced toward the blast and then walked in the other direction rapidly but calmly, it said. Ten seconds later, the second blast occurred, and evidence shows the backpack Mr. Tsarnaev left behind in front of the Forum restaurant is the only thing that could have caused the second blast, according to the affidavit.

On Thursday night, the two brothers stole a car at gunpoint from a man in Cambridge, Mass., authorities said. The brothers told the victim "that they were the Boston Marathon bombers and wouldn't kill him because he wasn't American," according to a report by the Cambridge Police Department. The victim was described as an Asian-American male.

Photos: Camaraderie After Boston Chaos

People gathered Sunday on Boylston Street at a memorial for victims of the Boston Marathon bombings. Nicolaus Czarnecki/Zuma Press

Timeline: Boston Bombings and the Search for Suspects

Map: Boston Area

Timeline: Terror in the U.S.

The FBI affidavit said that during the carjacking one of the brothers—the papers didn't specify which one—pointed a gun at the victim and said: "Did you hear about the Boston explosion?" He then added: "I did that…I am serious."

The man eventually escaped from the brothers, after letting them use his bank card to withdraw money from an automated teller machine, authorities said.

The victim, who hasn't been identified, told police that he heard the men mention "Manhattan'' while he was their captive, according to law-enforcement officials. The officials cautioned that they don't know yet whether the suspects were discussing New York, in large part because the two suspects were speaking Russian for much of the conversation and it is possible their victim misunderstood.

According to Britney Smith, a friend of Dzhokhar Tsarnaev, the young man visited New York City about eight months ago, and had fun hanging out with friends. During the manhunt for the brothers, Ms. Smith said FBI agents visited her mother's home on Long Island, out of concern the younger brother might try to visit her there.

Soon after the brothers' alleged carjacking, police began chasing them, culminating in a shootout in which the elder brother was killed. Dzhokhar Tsarnaev managed to escape, but he was badly injured and captured late Friday hiding in a boat being stored in a yard.

Federal investigators said that they suspect the two brothers may have been planning to launch another attack when they were chased by police, and that they had about a half-dozen additional explosive devices with them.

The indictment doesn't include some of the state-level offenses in which Mr. Tsarnaev is a suspect, including the murder of a Massachusetts Institute of Technology police officer and the wounding of another officer during a shootout. A spokeswoman for the Middlesex County District Attorney, which would handle those cases, said Monday that charges are expected soon.

The bombs used during the Boston Marathon, the federal affidavit said, were "low-grade explosives that were housed in pressure cookers" and packed with BBs, nails and green-colored hobby fuse.

After the shootout, police found homemade bombs that appeared to be of the same design and materials as the ones used in the attack at the marathon, the FBI said. At least one was a pressure cooker that contained BBs and nails, the affidavit said.

When agents searched his room at the University of Massachusetts Dartmouth, they found, among other things, "a large pyrotechnic, a black jacket and a white hat of the same general appearance as those worn" by the bomber identified as Dzhokhar Tsarnaev at the Boston Marathon, according to the court papers.

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